Is there any explanation to Bowser being able to change size?

Through the different games of the Mario series, we have seen Bowser changing size loads and loads of times.

Sometimes this change is shown and explained. That's the case in Super Mario World 2: Yoshi's Island and Yoshi's New Island, where Kamek use his magic to increase Bowser's size. In Super Mario Galaxy 2, his size apparently increase because of the magic of the power stars (even though his original size is already incredible).

But in Super Mario Sunshine, I didn't find any explanation about this. Bowser is just gigantic in the final battle. However, in the final cutscene, Bowser seems to be back to his original size again (you can totally compare his size with Bowser Jr. in both scenes).

Is there any explanation in a game (like a dialog or anything) for Bowser being able to change size like that? Did I miss something in the game?